1. Field of the Invention
The present invention relates to determining thermal properties of materials using a miniaturized resistive thermal probe. More particularly, the present invention relates to performing localized thermal analysis experiments whereby calorimetric information is obtained from a volume of materials on the order of a few cubic microns, whereas in conventional bulk calorimetry data is obtained from volumes of material on the order of a few cubic millimeters. The present invention also relates to modulating the temperature of a thermal probe to generate evanescent thermal waves in a material to thereby generate sub-surface images.
2. Background of the Invention
Several methods for the non-destructive characterization of solids make use of thermal excitation. Any local disruption of the structure that results in a change in density, specific heat, or thermal conductivity may be detected by some type of thermal probexe2x80x94often with higher sensitivity than by the use of optical, X-ray, or electron-microscope techniques. Many of these techniques use an intensity-modulated energy source to excite a sample. That is, the intensity of the energy source used to excite the sample is made to vary with time. Induced scattered evanescent thermal waves are then detected by, for example, monitoring the surface temperature of the sample. When a scanning mechanism is also incorporated, it is then possible to achieve spatial thermal mapping. Subsurface imaging can be performed within the depth of penetration of the thermal wave.
Most conventional methods of thermal imaging employ an energy beam that emerges from a small source and spreads out according to the rules of diffraction. The extent of this spreading is normally governed by the wavelength associated with the energy flux. However, if the sample is within the xe2x80x9cnear-fieldxe2x80x9d region, i.e., significantly less than one wavelength away from the source, then a greatly reduced beam diameter can be achieved. In this case, the beam diameter is not much larger than the size of the source itself.
This principle is applied in Scanning Probe Microscopy whereby a sharp probe is brought in close proximity to the surface of a sample. Some probe/sample interaction takes place. This interaction is monitored as the probe is scanned over the surface. An image contrast is then computer-generated. The image contrast represents variations of some property (e.g., physical, mechanical, chemical) of the sample across the scanned area. One such scanning probe microscope is the Atomic Force Microscope (AFM). In conventional AFM, the height of a probe above the surface being scanned is controlled by a feedback system, that keeps the force between the probe and the surface of the sample constant. The probe height is monitored, and provides the data that is used to create image contrast which represents the topography of the scanned area.
Near-field thermal imaging is described by C. C. Williams and H. K. Wickramasinghe in Photoacoustic and Photothermal Phenomena, P. Hess and J. Peltzl (eds.), p. 364 (1988). In their device, the probe is a specially made coaxial tip that forms a fine thermocouple junction. This probe provides a spatial resolution on the order of tens of millimeters. The sample is either heated using a laser or the probe, or the sample is heated electrically. The feedback system maintains the probe temperature constant (instead of maintaining the force constant), by varying the probe height as necessary.
In xe2x80x9cThermal Imaging Using the Atomic Force Microscope,xe2x80x9d Appl. Phys. Lett., vol. 62, pp. 2501-3 (1993), Majumdar, et al. describe a technique for thermal imaging that uses a simpler design of thermocouple tip, than that disclosed by Williams and Wickramasinghe. Majundar, et al. implemented standard atomic force microscopy feedback to maintain tip/sample contact. R. B. Dinwiddie, R. J. Pylkki and P. E. West xe2x80x9cThermal Conductivity Contrast Imaging with a Scanning Thermal Microscope,xe2x80x9d Thermal Conductivity 22, T. W. Tong (ed.) (1994), describe the use of a probe in the form of a tiny platinum resistance thermometer. U.S. Pat. No. 5,441,343 to Pylkki et al., which is incorporated herein by reference, discloses the thermal sensing probe for use with a scanning probe microscope, in which the contact force of the probe is maintained at a constant level as the probe is scanned across the surface of the sample.
In those studies, the samples were generally probed at a constant (ac or dc) amplitude of either surface temperature or heat flow. Thus changes in the thermal properties of materials, such as heat capacity or thermal conductivity, were not investigated. This is because the temperature of the sample was not raised by a sufficient amount for a change in the sample""s thermal properties to be detected.
Bulk thermal analysis techniques have been developed to study such changes in the thermal properties of materials. Modulated temperature differential scanning microscopy (MDSC) and spatially-resolved modulated differential scanning calorimetry (SR-MDSC) are described in U.S. Pat. No. 5,224,775 (""775 patent) and U.S. Pat. No. 5,248,199, respectively, which are both incorporated herein by reference. A conventional heat flux differential scanning calorimeter (DSC) measures the heat flow into and out of a sample with respect to a reference. Both sample and reference are usually subjected to a linear temperature/time ramp. In one implementation of MDSC, a sinusoidal modulation is superimposed on the underlying heating ramp to generate a corresponding sinusoidal response in the heat flow signal. This results in two measurements of heat capacity, an underlying linear long-period measurement due to the underlying heating ramp and a higher-frequency cyclic measurement due to the superimposed sinusoidal modulation. For many systems the cyclic measurement xe2x80x9cseesxe2x80x9d only the reversible heat capacity associated with molecular vibrations, e.g., glass transitions, whereas the underlying measurement also sees endotherms and exotherms associated with kinetically-controlled events such as recrystallization cure reactions or the loss of volatile materials.
The present invention is a new analytical technique which makes calorimetric measurements on a localized scale. Data obtained from the measurement can be used to generate contrast in our image of the thermal properties of the sample on a localized scale. In addition, by subjecting the sample to an oscillating program, images of the sample at a depth below the surface can be made. The depth corresponds to the frequency of the applied oscillatory temperature.
The present invention applies modulated temperature differential scanning calorimetry, as described in U.S. Pat. No. 5,224,755 to Reading, et al. (""775 patent), which has been conventionally used to perform bulk thermal analysis experiments of a sample material, to microscopic thermal analysis of a sample material using two highly miniaturized resistive probes, developed by the Topometrix Corporation (U.S. Pat. No. 5,441,343 to Pylkki et al. (""343 patent)), in a differential configuration. A sample probe attached to a Scanning Probe Microscope is positioned at a desired location on the surface within the field of view. Localized calorimetry is then performed at that position by inducing and detecting localized phase transitions. This is achieved by ramping the temperature of the probe by passing an electrical current through the probe. A small temperature oscillation is superimposed to that temperature ramp by adding a modulated component to current the probe current. By scanning over the surface of the sample, contrast can be developed corresponding to particular locations on the sample to create an image of the thermal properties of the sample at particular locations.
A second embodiment of the present invention allows for sub-surface imaging thermal microscopy to be performed by modulating the temperature of the probe. This is done by passing a modulated current through it, thus generating thermal waves within the sample. The depth of penetration of these waves is frequency dependent, such that the thermal properties of a sample can be probed as a function of depth below the surface.
The probe, developed by the Topometrix Corporation, is an elongated loop of Wollaston wire, shaped in the form of a canteliver whose end forms the resistive element. The resistance of that element varies with temperature. Conversely, its temperature can be set by passing a current of appropriate value through it. A mirror is attached across the loop allowing for the contact force of the element on the sample to be held constant, as in conventional atomic force microscopy, while the probe is scanned across the surface of the sample.
In the two embodiments of the present invention, the probe is used as a highly localized heat source by passing a current through it. The temperature of the probe is either constant, or is variable as a function of time. As the probe is brought close to the surface of a sample, heat will flow from the probe to the sample. The amount of heat flowing will vary according to various properties of the sample at the location under the probe. This varying heat flow causes the temperature of the resistive element to change, thereby changing its resistance. A feedback circuit is preferably used to sense the change in the probe resistance (and therefore its temperature) and increase the amount of current flowing through the probe to bring it back to its original resistance value (and therefore to its set temperature). A differential signal is then monitored, either directly or through a lock-in amplifier. The differential signal is used to either (1) produce localized analysis plots of amplitude and phase data versus temperature that provide calorimetric information at a specific position on the sample, or (2) construct an image whose contrasts represent variations in thermal conductivity and/or diffusivity across a scanned area. In the second case, the time-varying current through the resistive elements generates thermal waves in the sample. The modulation frequency of the time-varying current is functionally related to the depth below the surface of the sample at which an image of the sample is desired. A sub-surface image is thus generated. The depth of material below the sample surface that is contributing to the image can be controlled by suitably choosing the temperature modulation frequency. As described in Almond, et al., xe2x80x9cPhotothermal Science and Techniques,xe2x80x9d page 15, Chapman and Hall (London 1996), which is hereby incorporated by reference in its entirety, the penetration depth is proportional to the square root of the thermal diffusivity of the sample divided by the frequency of the applied temperature wave.
A first object of the present invention is to provide localized differential thermal analysis at the micron level, such that events such as glass transitions and meltings can be induced in a highly spatially localized manner and identified.
A second object of the present invention is to use scanning thermal microscopy to obtain sub-surface images of solids, in which the image contrast is due to variations in thermal diffusivity within a given controllable depth.
Another object of the present invention is to use a highly miniaturized resistive thermal probe as a highly localized source of heat, such that it can be used to produce highly localized phase transitions at the surface of samples.
Another object of the invention is to use modulated differential scanning calorimetric signals to produce a high-frequency temperature modulation in a probe used for sensing the thermal properties of materials at different depths below the surface.
Another object of the present invention is to provide an apparatus which can, for a sample containing different regions subject to either reversible or irreversible changes with temperature, produce thermal images showing contrast based upon the reversing and nonreversing nature of the heat flow, respectively.
These and other objects of the present invention are described in greater detail in the detailed description of the invention, the appended drawings and the attached claims.